1. Field of the Invention
This invention relates generally to cutting tools. More specifically, the invention concerns apparatus and a method for forming threads in a workpiece.
2. Description of the Prior Art
Conventional thread generation on workpiece surfaces, interior or external, is performed in one of two ways. In a first prior art approach, a single point cutting cutting element, or thread chaser insert, is used by performing multiple cuts on the workpiece surface with successive passes of the same single cutting tooth, each pass taking a deeper cut than the previous pass. Such an approach is time consuming and complicated by the necessity for accurately repositioning the cutting point with respect to the workpiece prior to each cutting pass.
A second prior art approach makes use of one or more multi-toothed cutting elements or thread chaser inserts, with each tooth of each such insert positioned to take a deeper cut than a preceding tooth. Hence, the time required for generating the same thread form is substantially reduced from that time required when using a single point thread chaser. However, for more complex thread forms having relatively tight tolerance requirements, the last, or deepest, cutting tooth on the multi-tooth chaser frequently is prone to fracture due to its required shape. Such damage, in turn, necessitates scrapping the entire expensive multi-tooth chaser, even though only a single cutting tooth thereof has been damaged.
The instant invention is directed to overcoming the disadvantages of both the above prior approaches by a novel combination approach utilizing the advantages offered by both old arrangements. The most pertinent prior art known to applicants is disclosed in U.S. Pat. No. 2,954,570--Couch, wherein a thread chaser tool includes means for releasably mounting a plurality of single point thread cutting tools in abutting, side by side relationship, and means for securing the various cutting tools in adjusted position relative to the workpiece. This prior art approach does overcome the disadvantage of having to replace an entire multi-toothed element whenever one tooth fractures. However, by using a separate single point element for every tooth of the threading tool, the Couch arrangement involves a great deal of required set-up time, and the holder is relatively expensive to produce, because of the plurality of threaded cutting element mounting bores and associated mounting hardware involved.